Unanswered Question about heaters, extruder

I have noticed that most extruder heaters have red wiring, but I have one with blue wiring that came with a V6 kit. Does the color indicate ampage usage?
Also, how can you tell the proper supply voltage. I have seen some listed as 12V and some listed as 24V.
There is also a difference of heater casing length. Hay, better to ask before a problem occurs.

 

E3D recently started shipping 25W heaters that have blue wires to replace the 40W with red.

You can tell the supply voltage only by measuring the resistance. Though if you have 24v heaters trying to run on 12v they'll be under powered and won't work very well. Though no damage will occur. The other way around might (12v heater on 24v supplied)

 

I was wondering because my hex all metal replacement has red wires and a 30W 12v rating.
I have also wondered about the thermistors. The only advantage I can see in a higher wattage heater is possibly faster warmup. The 30w unit keeps the block at app +- 1.5 degrees variation.
The hex came with a EPCOS thermistor and it works with no changing of settings.
I ran a 5 hour print and it looks good.